1. Field of the Invention:
The present invention relates to apparatus and methods for transferring data between a source and a plurality of receiving electrical modules. More particularly, the present invention relates to data transfer along a bus between a source data processing device and a plurality of electrical instrumentation modules.
2. Art Background:
In the computing industry, it is quite common to transfer data and commands between a plurality of electrical modules, such as for example, computers, printers, memories, as well as digitally controlled transducers and other sensing devices, on a system or data bus. Common constraints on the speed of data transfer between electrical modules are protocol or "handshake" restrictions which require a sequence of predetermined events to occur within specified time periods prior to the actual data exchange between modules. For example, U.S. Pat. No. 3,810,103 which issued on May 7, 1974 discloses the current IEEE-488 standard for data transfer interface protocol between various electrical modules such as computers, printers, and instrumentation devices. The structure and protocol of the IEEE-488 bus inherently slows data transfer between a source and receiving electrical module in that a relatively complex handshake sequence is required prior to actual data transmission.
A typical IEEE-488 data transmission between a source and a receiving electrical module requires that the sender initialize a DAV line to a high logic state such that the receiving module then sets a NRFD ("Not Ready For Data") line to low thereby permitting the source module to check for any error conditions prior to transmitting data on a data bus. After a predetermined time, the receiving module alters the state of the NRFD line indicating it is now ready to receive data. The source module, upon sensing the NRFD state change then alters the DAV line state to indicate to the receiving module that data on the data bus lines is now valid. The receiving module, upon sensing the DAV state change, accepts the data and alters the NRFD line to indicate that it is no longer ready to accept additional data, and sets a NDAC line to high to indicate that it has now accepted the data.
The source module, having sensed that the NDAC line is now high, sets the DAV line to high to indicate to the receiving module that the data on the data bus must now be considered not valid. Further data transmissions between the source and receiving module require that this complex handshake protocol be repeated for each transmission. Moreover, in the cas where the IEEE-488 standard is used to interconnect both programmable and nonprogrammable electronic instrumentation measuring apparatus with other data processing equipment or accessories, the IEEE-488 bus is address "dependent" in that each instrumentation module coupled to the IEEE-488 bus is given a N address such that its position on the bus is unique. In other words, in order to relocate an instrumentation module to another position on the IEEE-488 bus, it is necessary to redefine the address location for the particular module.
Accordingly, in applications where it is necessary to communicate between a source data processing apparatus and a plurality of instrumentation modules, such as for example digital voltmeters, analog-to-digital (A/D) converters, temperature sensing devices, passive digital switches, programmable power supplies and the like, the existing IEEE-488 standard with its complex data transfer protocol has been found to be unacceptably slow and cumbersome.
As will be described more fully below, the present invention provides a bus and data transfer method having particular application to test measurement and instrumentation systems. The present invention incorporates a bus which permits communication between a source data processing apparatus and a plurality of instrumentation modules and external electrical control devices, and permits high speed data transfer using a minimum of transfer operation events prior to actual data transmission.
Although the present invention has particular utility with respect to instrumentation and test measurement systems, it will be appreciated that the disclosed bus structure and method may be used in numerous data transfer applications.